1. Technical Field
The present invention relates generally to satellite communication networks, and more particularly to a methodology for managing operation of a low earth orbit satellite (LEOS) system.
2. Related Art
Satellite communication systems are generally known to facilitate wireless communications across most areas of the earth""s surface. Satellite communication systems may provide wireless coverage for mobile subscriber units in areas that are served neither by the public switched telephone network (PSTN) nor by cellular communication systems. The satellite communication systems may also provide a high bandwidth routing path for voice and data communications.
In a satellite communication system, at least one satellite operates from an orbit above the earth""s surface. International telecommunication satellites typically operate from a geo-stationary orbit (GEO) that is approximately 36,000 kilometers above earth. Alternatively, telecommunication satellites have also been implemented in orbits closer to earth in low earth orbit satellite (LEOS) systems at an altitude from 700 kilometers to 2,000 kilometers or medium earth orbit satellite (MEOS) systems at approximately 10,000 kilometers.
In a LEOS system, a plurality of satellites orbit the earth and together provide coverage across most areas of the earth""s surface. As contrasted to a GEO satellite system in which a single satellite provides coverage over a substantial geographic area for all times, in a LEOS system the coverage area of each orbiting satellite changes over time. Thus, a first LEOS system satellite will provide coverage for a geographic area during a first time period while another LEOS system satellite provides coverage for the geographic area for a second time period, etc.
In a typical LEOS configuration, satellites are organized into orbital planes. A plurality of satellites orbits the earth in the orbital plane such that the satellites pass near the south pole and the north pole during their orbits. The satellites of each orbital plane are substantially uniformly distributed about the orbital plane and, as a whole, provide coverage for a geographic area corresponding to the orbital plane. By providing a plurality of orbital planes, each of which is separated from adjacent orbital planes by a separation angle, communications across a significant portion of the earth""s surface are supported.
The number of orbital planes, the number of satellites in each orbital plane, the separation of the orbital planes, the altitude of the satellites in each orbital plane, and the inclination of each orbital plane characterizes the xe2x80x9cconstellationxe2x80x9d of the LEOS system. The constellation essentially describes the relative positions and motion of the satellites in the LEOS system.
The LEOS system is called upon to service terrestrial communications between sources and destinations. Sources and destinations are devices that couple communications to the LEOS system. A source or destination may be a wireless subscriber unit, an earth station, which couples the LEOS system to the PSTN or another communication network, or any other wireless device that couples to one of the satellites.
In a LEOS system, communications are not only coupled from sources and destinations to satellites, but they may be coupled between satellites as well via intersatellite links (ISLs). For the purposes of overall communication path length evaluation, each ISL may be referred to as a xe2x80x9chop.xe2x80x9d Each hop consumes LEOS system resources and adds delay to the communication. Thus, it is desirable to minimize the number of hops within the LEOS system when coupling communications between sources and destinations. However, the number of hops between a first satellite servicing a source and a second satellite servicing a destination depends upon system routing behavior and the positions of the source and destination. System routing behavior is based not only upon programmed permissible ISLs between satellites but upon the LEOS system constellation as well.
Once a LEOS system is built and the satellites are deployed, it must be efficiently operated in order to maximize system capacity and to meet a desired grade of service. During normal operation, because satellites orbit the earth and alter their relative positions with respect to one another, ISLs are established and broken on a regular basis. With hundreds of satellites deployed in a typical LEOS system, however, determining which ISLs to establish, when to establish ISLs and when to break the ISLs is a difficult task.
Thus, there is a need in the art for a methodology for managing ISLs in a LEOS system so that the available resources of the LEOS system are efficiently used to adequately service communication load.
To overcome the shortcomings of the prior systems and their operations, a method according to the present invention manages inter-satellite links (ISLs) in a LEOS system. According to a first aspect of the present invention, intra-plane ISLs are established between a subject satellite and adjacent satellites in the same orbital plane. These intra-plane ISLs are generally fixed during all operations and will not be severed or reestablished once they have been initially established.
According to another aspect of the present invention, neighbor ISLs (i.e., ISLs between a subject satellite in a first orbital plane and neighbor satellites in an adjacent orbital plane) are established to minimize offset. While the distance of the ISLs may not be minimized for all times, by selecting neighbor ISLs to minimize offset, the links may be more easily selected and managed. Further, neighbor links are coordinated between a subject plane and a neighbor plane to eliminate loops and inefficient routing paths.
In managing neighbor ISLs according to the present invention, a reference satellite is selected. The offset to three neighboring satellites in a neighbor orbital plane is then determined. Of these three offsets, the two smaller offsets (and corresponding satellites) are selected. System conditions will dictate when the neighbor ISLs for a particular orbital plane will be altered. System load, satellite loading and satellite coverage may all be considered in determining whether to alter the neighbor ISLs. When it is determined that neighbor ISLs are to be altered for an orbital plane, the neighbor ISLs for all satellites in the orbital plane will be altered in a coordinated fashion. In another operation, satellites in the neighbor plane may be selected for neighbor ISLs which do no currently provide the minimal offset position but soon will based upon the continuing relative motion between the planes.
According to still another aspect of the present invention, seam ISLs are managed. When a satellite operates in a teardrop area (area in which the satellite provides some coverage), the satellite establishes seam ISLs to up to two seam satellites in a seam orbital plane. However, when the satellite moves into a mesh area, it may relinquish all seam ISLs. When such operation occurs, an ISL between the seam satellite previously having a link to the subject satellite may be established to a satellite in an adjacent plane. By removing the subject satellite from the ISL routing when it has no coverage area, communications are more efficiently routed and routing delays are reduced.
Other aspects of the present invention will become apparent with further reference to the drawings and specification that follow.